A method called an MS/MS analysis (or tandem analysis) is widely used as one of the mass spectrometric techniques for identification, structural analyses or quantitative determination of compounds having large molecular weights. There are various kinds of mass spectrometers with different configurations designed for the MS/MS analysis, among which tandem quadrupole mass spectrometers are characterized by their relatively simple structure as well as easy operation and handling.
In a generally used tandem quadrupole mass spectrometer, ions generated from a sample in an ion source are introduced into a first quadrupole mass filter (which is often represented as “Q1”), in which an on having a specific mass-to-charge ratio (m/z) is selected as a precursor ion. This precursor ion is introduced into a collision cell containing an ion guide with four or more poles (this ion guide is commonly represented as “q2”). A collision-induced dissociation (CID) gas, such as argon, is supplied to this collision cell, and the precursor ion in the collision cell collides with this CID gas, to be fragmented into various kinds of product ions. These product ions are introduced into a second quadrupole mass filter (which is often represented as “Q3”), which selectively allows a product ion having a specific mass-to-charge ratio (m/z) to pass through it and reach a detector, to be thereby detected.
The tandem quadrupole mass spectrometer can be used independently. However, this device is often coupled with a chromatograph, such as a gas chromatograph (GC) or liquid chromatograph (LC). In recent years, chromatograph tandem quadrupole mass spectrometers have become vital devices in the field of analyzing a trace amount of a sample containing a large amount of compounds or a sample contaminated with various impurities, such as testing residual pesticides in foodstuffs, testing environmental pollutants, checking the concentration of medicinal chemicals in blood, or screening drugs or poisonous substances.
MS/MS analyses by chromatograph tandem quadrupole mass spectrometers can be conducted in various measurement modes, such as a multiple reaction monitoring (MRM) mode, precursor-ion scan mode, product-ion scan mode, and neutral-loss scan mode (see Patent Document 1). In the MRM mode, the mass-to-charge ratio at which ions are allowed to pass through is fixed in each of the first and second quadrupole mass filters so as to fragment a specific kind of precursor ion and measure an intensity (or amount) of a specific kind of product ion resulting from the fragmentation. The two-stage mass filtering in the MRM measurement eliminates unwanted components other than those to be analyzed, ions originating from impurities, and neutral particles, so that an ion intensity signal with high signal-to-noise ratio can be obtained. Due to this feature, the MRM measurement is particularly effective for the quantitative analysis of a trace amount of a component. For example, gas chromatograph tandem mass spectrometers (GC/MS/MS) are frequently operated in the MRM mode to perform a simultaneous mufti-component quantitative analysis of residual pesticides, which requires determining the quantity of an extremely small amount of components.
To ensure a high accuracy of quantitative determination by the MRM measurement, it is necessary to set appropriate MRM measurement parameters for each compound, such as the mass-to-charge ratio of a precursor ion, the mass-to-charge ratio of a product ion and the collision energy. In the case of quantitative analyses, a mass-to-charge ratio corresponding to the maximal intensity of the ions originating from the compound in question is selected as the aforementioned mass-to-charge ratio. An ion having the thus selected mass-to-charge ratio is called the target ion. If there is a foreign component which has a peak at the same mass-to-charge ratio as that of the target ion on a mass spectrum and which has roughly the same retention time as the target ion, it is difficult to distinguish the target compound from the foreign component by only the target ion. In one commonly used solution to this problem, an ion which originates from the target compound but has a different mass-to-charge ratio is designated beforehand as a qualifier ion, and the relative ratio of the peak intensity corresponding to the qualifier ion to the peak intensity corresponding to the target ion on a mass spectrum representative of the peak located on the mass chromatogram of the target ion is calculated. If this ratio (“qualifier ion ratio”) is within a predetermined range, the peaks on the mass chromatogram can be considered to have originated from the target compound.
To determine the optimal target ion, qualifier ion or collision energy for each compound, an MS/MS analysis using a chromatograph tandem quadrupole mass spectrometer is performed on a sample containing a target compound a plurality of times while varying the collision energy. Then, a plurality of mass spectra (product-ion spectra) corresponding to different collision energies obtained in the vicinity of the retention time at which the target compound should appear are compared to determine an optimal target ion, qualifier ion, collision energy, and so on. Such a series of operations and determinations are performed by an analysis operator.
That is to say, in a conventional data-processing system for chromatographic tandem quadrupole mass spectrometry, after measurement data by an MS/MS analysis are collected, when the operator selects a set of measurement data that should be re-analyzed, the selected data are read from a storage, and a total ion chromatogram is created from those data and shown on a display screen. On this total ion chromatogram, the operator selects a peak or point that seems to correspond to the target compound, by means of a pointing device or other kinds of input devices. Then, a mass spectrum actually measured at the point in time corresponding to the selected point is created and shown in a different area on the display screen. Subsequently, when the operator similarly selects another set of measurement data obtained under a different amount of collision energy, the selected data are read from the storage, and another mass spectrum is created from the data and shown on the same display screen. Thus, a plurality of mass spectra obtained under different amounts of collision energy can be simultaneously shown on the same display screen. By comparing the displayed mass spectra, the operator can select an appropriate target ion and qualifier ion as well as appropriate collision energy.
If the sample contains only the target compound, the previously described process causes no problem. Actually, samples used for such purposes normally contain a plurality of compounds so as to allow parallel determination of the measurement parameters for multiple compounds. Therefore, operators need to check whether or not a peak located on the total ion chromatogram corresponds to an intended target compound. For that purpose, it is necessary to refer to another type of information, such as a compound table. In the case of simultaneous multiple-component quantitative determination, the aforementioned checking is required for each of a large number of compounds. This work is cumbersome and lowers the working efficiency.
The previously described problem occurs not only in the determination of MRM measurement parameters used in a chromatograph tandem quadrupole mass spectrometer, but also in other situations, such as the determination of the voltage settings of an ion transport optical system used in a chromatograph quadrupole mass spectrometer.